The present invention relates to a process for producing pyromellitic anhydride from a tetraalkylbenzene and/or a trialkylbenzaldehyde by the catalytic gas-phase oxidation method.
Pyromellitic anhydride is a compound of value as various industrial starting materials, finding application in heat-resistant resins, plasticizers, curing agents for epoxy resin, and so on. For the production method of pyromellitic anhydride, several processes have heretofore been proposed, and for the catalytic gas-phase oxidation of 1,2,4,5-tetraalkylbenzenes, for instance, relevant technologies are disclosed in Japanese Kokoku Publication Sho-49-9451 and Japanese Kokoku Publication Hei-4-15020, among others. Furthermore, the process for liquid-phase oxidation of 1,2,4,5-tetraalkylbenzenes (Japanese Kokai Publication Sho-61-27942), the process for liquid-phase oxidation of 2,4,5-trimethylbenzaldehyde (Japanese Kokai Publication Sho-57-38745), and the catalytic gas-phase oxidation process starting with anthracene (Japanese Kokai Publication Sho-56-8388) have also been disclosed.
Regarding the catalyst for the production of pyromellitic anhydride by the catalytic gas-phase oxidation, the following catalysts, among others, have been disclosed: V2Oxe2x80x94TiO2, WO2 type (Belgian Patent 655686), V2O5xe2x80x94P2O5xe2x80x94TiO2, MoO3, WO3(Japanese Kokoku Publication Sho-45-4978), V2O5xe2x80x94TiO2(anatase-type) xe2x80x94MoO3, P2O5(Japanese Kokoku Publication Sho-45-15018), V2O5xe2x80x94TiO2xe2x80x94Na2Oxe2x80x94P2O5 type (Japanese Kokoku Publication Sho-45-15252), V2O5xe2x80x94MoO3xe2x80x94P2O5 (Japanese Kokoku Publication Sho-47-38431), V2O5xe2x80x94TiO2xe2x80x94P2O5xe2x80x94Nb2O5xe2x80x94K2O, P2O5, TiO2, Na2O (Japanese Kokoku Publication Sho-49-31973), V2O5xe2x80x94B2O5 (Japanese Kokoku Publication Sho-48-35251), V2O5xe2x80x94Na2Oxe2x80x94MoO3xe2x80x94Cr, Mn, Nb (Japanese Kokai Publication Hei-1-294679), V2O5xe2x80x94WO3xe2x80x94TiO2(rutile-type) xe2x80x94Mn, Sb, Te, Bi, P, Cu, Al, Group VIIIB, alkali metal (Japanese Kokai 2000-79344).
Furthermore, referring to the mode of use of the catalyst for the production of pyromellitic anhydride, particularly to examples in which the catalyst bed is divided into two or more stages, Japanese Kokoku Publication Hei-4-15020, Japanese Kokai Publication Hei-1-245857 and Japanese Kokoku Publication Hei-4-13026, for instance, disclose technologies for suppressing the reaction in the hot spot of a catalyst bed to thereby reduce the temperature of said hot spot, which comprise either diluting the catalyst with a carrier, increasing the catalyst diameter, reducing the supported amount of the catalyst, adjusting the vanadium, alkali metal or phosphorus content of the catalyst, or reducing the specific surface area of ZrO2, TiO2 or SnO2. Japanese Kokai Publication Hei-8-41067 discloses the method comprising using a Mo-supplemented catalyst for the catalyst bed on the reaction gas outlet side, or a catalyst supplemented with a large amount of an alkali metal for the catalyst bed on the reaction gas inlet side to improve the yield of pyromellitic anhydride.
Furthermore, as the reaction technology for the production of pyromellitic anhydride, Japanese Kokoku Publication Sho-63-7537 discloses a method of increasing the product yield which comprises adding water vapor to the reactant gas to be introduced into the catalyst bed.
Although pyromellitic anhydride can be produced by such various alternative techniques as mentioned above, it was insufficient concerning the study regarding the catalyst life. In Japanese Kokoku Publication Sho-44-29446it is stated that a 30-hour intermittent reaction entailed a yield reduction of 1.9 weight %. According to Japanese Kokai Publication Sho-61-25642, a one-month-long operation under low-oxygen concentration, high moisture content, high reaction temperature conditions resulted in a yield reduction of 0.9 mole %, and Japanese Kokai Publication Hei-11-104497 states that a 3-month-long operation resulted in a yield reduction of 0.2 mole %, 2.5 mole %. To cope with the above problem, Japanese Kokoku Publication Sho-45-4978 teaches a combination of V with P, Ti, W and Mo as a means for improving the aging of vanadium oxide type catalysts and states that this measure proved effective in a 30-hour intermittent operation. Japanese Kokoku Publication Sho-49-30821 referring to a supported Vxe2x80x94Pxe2x80x94Mo catalyst as immobilized on a molten alumina carrier, discloses that addition of Ti suppressed the yield loss to 0.5 mole % in a 1500-hour operation. However, even these techniques are inadequate in the effect of suppressing catalyst aging and have room for further improvement in the curtailment of the interval between catalyst changes. Thus, in commercial production, catalyst change is a significant factor leading to increased production cost owing to downtime and gain in catalyst cost so that a further improvement in catalyst life is required.
Meanwhile, in the production of pyromellitic anhydride, it was known until recently that the water vapor content of the starting material gas influences the reaction to enhance the yield of pyromellitic anhydride (PMDA) in the early stage of reaction (Japanese Kokoku Publication Sho-63-7537). However, there is no report on the effect which the water vapor content of the starting material gas may have on the time-dependent change of yield.
Developed in the above state of the art, the present invention has for its object to provide a production method of a pyromellitic anhydride which is not only conducive to an increased catalyst life and, hence, an enhanced operating rate of a plant (reduced downtime) and a reduction in catalyst cost but also conducive to reductions in byproducts and, hence, savings in the cost of after-treatments such as collection and purification.
The inventors of the present invention took note of the fact that change of the fixed-bed catalyst in the fixed-bed type reactor is so costly and time-consuming that the influence of catalyst life on economics is remarkable and further that in the case where the aging of the catalyst is severe, operating parameters must also be modified frequently with the result that even the operation itself may become difficult. Accordingly, we dared to explore into the aging behavior of the catalyst at a low water vapor concentration which is usually a cause for poor initial yield and found that the water vapor concentration exerts a profound influence on catalyst aging. As a result, we established a technology promising a high yield when the catalyst is used for at least 2000 hours and a still greater advantage when the operation is further prolonged. Thus, the inventors paid attention to the reaction parameters for a production method of a pyromellitic anhydride comprising a step for catalytic gas-phase oxidation of a starting mixture gas consisting of a tetraalkylbenzene and/or a trialkylbenzaldehyde and a molecular oxygen-containing gas in a fixed-bed type reactor having a catalyst bed and investigated them with great care and, as a result, found that the moisture content (the water vapor content) of the starting mixture gas to be introduced into the catalyst bed has a significant effect on catalyst aging and have succeeded in prolonging the catalyst life by reducing said content of water vapor to a certain level or below. We discovered, at the same time, that, by reducing the content of water vapor to such a low level, the formation of byproducts can also be suppressed to neatly resolve the above-mentioned problems. The present invention has been developed on the basis of the above findings.
The present invention, therefore, is concerned with a production method of a pyromellitic anhydride comprising a step for catalytic gas-phase oxidation of a starting mixture gas consisting of a tetraalkylbenzene and/or a trialkylbenzaldehyde and a molecular oxygen-containing gas in a fixed-bed type reactor having a catalyst bed
wherein the production method of a pyromellitic anhydride is carried out under the condition that the moisture content of said starting mixture gas to be introduced into the catalyst bed is not more than 2 volume %.
The present invention further relates to a production method of a pyromellitic anhydride comprising a step for catalytic gas-phase oxidation of a starting mixture gas consisting of a tetraalkylbenzene and/or a trialkylbenzaldehyde and a molecular oxygen-containing gas in a fixed-bed type reactor having a catalyst bed
wherein the production method of a pyromellitic anhydride is carried out by dehumidifying the molecular oxygen-containing gas.